4.1
General
(1) Due to the fact that this Specification embraces a wide range of product types and complexities, some of the differences are addressed as follows:-
(a) Some requirements of this Section necessitate a more stringent approach to testing for certain types of products than for others. The most significant example is the elecro-acoustic requirements of telephony devices (including cordless telephones) where normally 3 samples of each type shall be assessed.
(b) If the test results on 3 samples are too widely spread and/or any individual sample is too close to one of the limits, then tests of further samples may be called for.
* Sampling and evaluation principles are given in Appendix 1.
* For more complex products which have a telephony facility, the requirement for testing 3 samples may be waived at the discretion of Access Standards and then only one sample need be tested.
(c) Telephony devices fitted with user-controlled receive volume control shall comply with all mandatory requirements over the whole range of adjustment settings.
* Due to inherent noise and possible crosstalk on analogue lines, it is undesirable for the range of receive gain to be excessive. The typical range for gain is 6-10 dB.
(d) Any telephony device equipped with loudspeaking or 'handsfree' facilities in addition to a handset shall comply with this Specification. However, only the handset function is normally required to be tested.
* Loudspeaking or handsfree functions are normally treated as market features. Telephony products without handset facilities will each be treated on their individual merits and requirements should be discussed with Access Standards.
(e) There are no mandatory requirements for telephones to be capable of operating with a hearing aid.
* The capability to operate with inductive couplers and other such devices is regarded as a marketing feature. However, where inductive coupling is provided, ITU-T Recommendation P 37 should preferably be complied with.
* Telephony devices with inductive couplers should preferably be marked with the standard graphic symbol ref. NZS 4121:1985, AS 1428.2:1992 and ITU-T Rec. E.121.
(f) For cordless devices, the allocation of radio frequencies is administered by the Radio Operations Division of the Ministry of Commerce (ref. clause 1.1(4)).
(g) Any non-voice equipment capable of multiple speeds and modulations, such as a modem, shall comply with the requirements of this Section. The device shallbe tested at each combination of transmit bit rate and type of modulation available. Where the transmit rate is continuously variable, the highest bit rate shall be used.
(2) For a series connected device providing a proprietary interface for a terminal device, the combination of terminal and series connected devices shall comply with the applicable requirements of this Section.
* A typical example is a PABX with a system dependent extension phone.
(3) Except where otherwise stated, the test set up for transmission testing shall be as indicated in Fig. 4-1.
FIG. 4-1(a) TEST SET-UP FOR TRANSMISSION TESTS
FIG. 4-1(b) ARTIFICIAL LINE SEGMENT (0.1 km Line Length)
4.2
(a) Signals of more than 5 seconds duration shall not exceed a level of -9 dBm when averaged over 1 minute.
Transmitted power limitations
(1) Signals transmitted to line from any device shall not exceed the following limits:-
* Special consideration may be given for a relaxation of this limit to a level between -4 dBm and -9 dBm for signals of between 5 and 30 seconds duration. Such cases must be fully represented giving all details, and each case will be treated on its individual merits.
(b) Signals of less than 5 seconds duration shall not exceed a level of -4 dBm.
* Speech signals from a telephone instrument conforming to this Specification will, typically, have a nominal level of -12 dBm measured against 600 ohms when averaged over three seconds.
* Automatically generated DTMF signals are subject to this requirement (see clause 5.2.2). Manually generated DTMF signals, since they are under the direct control of the user, cannot be easily restricted or measured. However, it is assumed that any individual DTMF digit is unlikely to exceed 5 seconds.
(c) The maximum instantaneous peak power of any transmitted signal measured across an impedance of 600 ohms shall not exceed a level that corresponds to that of a sine wave of 0 dBm.
* Transient voice peaks may be as high as +12 dBm. These, because they are very brief, do not normally exceed the limit in (1)(c) above.
(2) On any telephony device, it shall not be possible to generate a signal exceeding +10 dBV rms across a 600 ohm termination for any acoustic stimulus at the microphone.
* This requirement relates to the limit set on line signals such that potential interference to other Telecom customers due to crosstalk is minimised.
(3) Any recorded message or voice signal transmitted into the Telecom PSTN shall be between the limits of -6 VU and -16 VU when measured using the test arrangement shown in Fig. 4-1(a) on a 0 km line. The preferred range is -6 VU to -12 VU.
(4) For any device capable of transmitting pre-recorded music, synthesised music or composite speech and music signals, the level of music or composite signals shall be between the limits of -12 VU and -25 VU when measured using the test arrangement shown in Fig. 4-1(a) on a 0 km line.
* See Appendix 2 for further information on 'VU' (volume unit) measurements. Return to Contents
4.3
Noise and crosstalk
4.3.1
Equipment on-hook
(1) The idle state or on-hook psophometric noise power level, measured against
600 ohms, of any device connected to a Telecom line shall be no greater than -65 dBmp.
* Mean psophometric noise power should be measured in accordance with CCITT Blue Book, Rec. O. 41.
* On-hook noise may affect the perceived performance of other telephones used on the same line. Also, excessive noise levels on one line may induce unwanted noise on adjacent lines.
(2) (a) The power level of individual spectral components above 4 kHz should not exceed the limits formed by straight lines joining the following coordinates
(see Fig. 4-2):-
| FREQUENCY (kHz) | POWER SPECTRAL DENSITY (dBm) |
|---|---|
| 4.0 | -40 |
| 10.0 | -40 |
| 20.0 | -50 |
| 50.0 | -70 |
| >50.0 & <100.0 | -70 |
| >100.0 & <10 MHz | -50 |
(b) The one minute mean power level in any 3 kHz bandwidth contained wholly above 4 kHz shall be no greater than the limits for the centre frequency of the band as determined in (a) above.
FIG. 4-2 NOISE POWER SPECTRAL DENSITY LIMITS
(3) For telephony devices, any coupling between the microphone and/or receiver, and the line, shall be such that the requirement of sub-clause (1) above is not exceeded when the room noise or conversation adjacent to the device is at a level of 65 dB SPL.
* This requirement relates to potential "bugging", whereby the telephone in its on-hook state could provide a source of signals to an unlawful listener.
* dB SPL is the sound pressure level in dB referenced to 20 µPa. Return to Contents
4.3.2
Equipment off-hook
(1) Non-signalling (call established) state:-
(a) The off-hook psophometric noise power level, generated by locally powered telephony devices when in the non-signalling state shall not exceed -60 dBmp, with the microphone rendered inoperative.
* Mean psophometric noise power should be measured in accordance with CCITT Blue Book, Rec. O. 41.
* A wide variation in ambient noise conditions may exist in any customer's premises and such noise is often at far higher levels than the electrical noise generated within modern terminal equipment.
(b) For devices transmitting recorded speech or music, the combined recording and playback facility shall have a signal-to-noise ratio of at least 20 dB over the frequency range 100 Hz to 5 kHz.
(c) For non-voice devices, the presence of unwanted signals in the frequency range 0 - 4 kHz is regarded as a marketing feature.
(d) For all devices, the noise requirements at frequencies above 4 kHz shall
be as stated for the on-hook state in clause 4.3.1(2) above.
(2) During signalling (call set-up phase for all devices):-
(a) The individual power level of any extraneous frequency in the range 300 Hz to 5 kHz during signalling shall be at least 20 dB below that of the signalling frequency with the lowest level.
(b) The one minute mean power level in any 3 kHz bandwidth contained wholly above 5 kHz shall be at least 20 dB below that of the signalling frequency with the lowest level.
* "Noise" includes any unwanted signals such as distortion and intermodulation products that may occur during call set-up signalling. Return to Contents
4.3.3
Multi-line devices
Any device having direct access to more than one line shall have a minimum of 50 dB crosstalk isolation between individual lines over the frequency range 300 - 3400 Hz. This shall apply to both on-hook and off-hook conditions.
* This requirement applies mainly to devices such as two-line telephones and some simple key systems. Requirements for products such as PABX's are covered separately in Specification PTC 109. Return to Contents
4.4
Distortion
(1) Send harmonic distortion:-
(a) The total send harmonic distortion of any telephony device should not exceed 5% for a pure tone signal of -5 dBPa at a frequency of 500 Hz with line currents of 20 mA and 80 mA.
(b) The total send harmonic distortion of any device transmitting recorded speech shall not exceed 10% for frequencies in the range 300 Hz to 3400 Hz.
(c) The total send harmonic distortion introduced by any non-terminating series connected device shall not exceed 5% on a 1 kHz signal applied at a level of
-4 dBm.
* The requirement stated in (c) above is designed to ensure that any distortion of signals, including DTMF, applied by the associated terminating device is within acceptable limits after passing through the series connected device. Hence the send test level of -4 dBm which is the maximum permissible send level for DTMF tones from terminating devices (see clause 5.2.2). (2) The total receive harmonic distortion of any telephony device should be less than 6% for a pure tone signal of -10 dBV at a frequency of 500 Hz with line currents of 20 mA and 80 mA.
(3) The total sidetone harmonic distortion of any telephony device should be less than 10% for a pure tone signal of -5 dBPa at a frequency of 500 Hz with line currents of 20 mA and 80 mA.
(4) For digital PABX/KTS systems, distortion shall comply with the requirements of Specification PTC 109, Section 7. Return to Contents
4.5
Line impedance of the device
4.5.1
Off-hook line impedance
(1) The objective off-hook impedance of any device, measured at the line terminals, is complex and is represented by a network consisting of a 370 ohm resistor in series with a parallel combination of a 620 ohm resistor and a 310 nF capacitor (see Fig. 4-3). This network is known as 'BT 3' for convenience .
* This impedance network was originally used by British Telecom.
* The impedance of a telephone instrument is also of extreme importance when connected to any private network or PABX. Incorrect impedance can result in excessive sidetone, echo, and/or instability problems.
FIG. 4-3 IMPEDANCE REFERENCE NETWORK (BT 3)
(2) Apart from the exceptions described in sub-clauses (4) to (8) below, the minimum return loss of the line port of any device against the impedance BT 3 shall be no lower than the following limits for line currents of 20 mA and 80 mA:-
(b) 12 dB at any frequency in the band 1000 - 3400Hz.
(3) Apart from the exceptions described in sub-clauses (7) and (8), for telephony devices the echo return loss against the impedance BT 3 shall be no less than 14 dB.
(a) 10 dB at any frequency in the band 300 - 1000 Hz.
* Echo return loss is derived from the return loss at individual frequencies, using the same formula as that described in ITU-T Recommendation G.122:1993 Annex B.4 for the derivation of "echo loss".
(4) For non-voice devices, the impedance requirements are relaxed from those stated above and are as stated in sub-clause (5) below. However, where a device meets only these relaxed requirements, the following warning notice shall be included in the User Manual:
"This equipment does not fully meet Telecom's impedance requirements.
Performance limitations may occur when used in conjunction with some parts of the network. Telecom will accept no responsibility should difficulties arise in such circumstances"
(5) The relaxed requirements for the minimum return loss of non-voice devices are as follows, subject to the condition stated in sub-clause (4) above:-
(a) Minimum return loss, measured against the reference impedance "BT3",
shall be at least 8 dB at any frequency in the band 300 to 3400 Hz, or
(b) minimum return loss, measured against an impedance of 600 ohm, shall be at least 16 dB at any frequency in the band 300 to 3400 Hz, or
(c) minimum return loss, measured against a reference network made up from a 220 ohm resistor in series with a parallel combination of an 820 ohm resistor and a120 nF capacitor, shall be at least:-
(i) 10 dB at any frequency in the band 300 to 600 Hz, and
(ii) 15 dB at any frequency in the band 600 to 3400 Hz.
* The impedance described in (c) is the ACA objective requirement.
* Suppliers should note that any equipment complying only under these relaxed conditions may give trouble when used in conjunction with some parts of the network. Telecom will accept no responsibility should difficulties arise in such circumstances. Suppliers are advised to monitor the performance of their equipment supplied under the relaxations of this Specification and to advise their customers where there is reason to believe that such problems will occur.
(6) Non-voice devices operating over a limited bandwidth (e.g. 1000 Hz to 2500 Hz) shall have minimum return loss of 3 dB against BT3 at frequencies outside the operating band. This relaxation is conditional on any signal transmitted outside the operating band being less than -30 dBm.
(7) Where a "hold" facility is provided on any type of device, the echo return loss against BT3 during activation of the hold condition shall be no less than 3 dB.
(8) While DTMF signals are being transmitted during the setting-up of a call, the minimum return loss against BT3 shall be no less than 5 dB.
* This relaxation does not apply to the transmission of DTMF signals following establishment of the call, e.g. signalling CPE to CPE. Return to Contents
4.5.2
On-hook line impedance
The on-hook impedance (including impedance of bridging equipment) of any device connected to a Telecom line shall be not less than 10 kohm for the frequency range
300 Hz to 3400 Hz when measured with 1 Vrms applied.
* This requirement may be relaxed for certain devices where it is known that they will definitely not be connected in parallel with any other device.
* Relaxation of this requirement may also be considered for devices using on-hook data transmission (see Section 11).
4.6
Port impedances of series connected devices
* See clause 2.12 for general information on series connected devices.
* See also clause 4.1(2) for series connected devices with proprietary interfaces. Return to Contents
4.6.1
(2) If the terminal equipment port of a series connected terminating device is 2-wire and provides a separately powered line feed to other terminal devices, the port impedance shall be nominally BT3 and shall satisfy the requirements of clause 4.5.1 above.
Terminating devices
(1) For series connected terminating devices, including those containing a 4-wire element (see Fig. 4-5), the PSTN line port shall be as specified in clause 4.5.1(2) and (3) above with a reference BT3 network connected to the terminal equipment port.
4.6.2
(a) The return loss at the line port of the series device shall be measured
against BT3 for line currents of 20 mA and 80 mA, with the terminal equipment port terminated in a reference BT3 impedance (see Figs. 4-4 and 4-5).
Non-terminating devices
For a non-terminating series connected device designed for connection between the line and a separate terminal device, the return loss requirements are as follows:-
* If the series device is dependent upon the d.c. line feed for it's operation, to test the return loss it may be necessary to also provide a d.c. termination with the BT3 a.c. impedance.
(b) The return loss measured shall be no less than 25 dB at any frequency in the band 300 - 3400 Hz.
FIG. 4-4 2-WIRE SERIES CONNECTED DEVICE
FIG. 4-5 4-WIRE SERIES CONNECTED DEVICE
4.7
Impedance of bridging equipment
See clause 4.5.2.
Return to Contents
4.8
Impedance balance ratio to earth
(1) Devices which have a direct or indirect connection to earth shall, for both on-hook and off-hook conditions, have an impedance balance ratio to earth of not less than
40 dB over the frequency range 200 - 4000 Hz when tested as shown in Fig. 4-6.
(2) For voice type equipment it is strongly recommended that, in the off-hook condition, devices should also have an impedance balance ratio to earth of not less than 60 dB over the frequency range 200 - 1000 Hz.
(3) Equipment connected directly or indirectly to mains power supplies shall also comply with the requirements of this clause.
* ELV plug packs will provide some capacitive coupling between earth and the ELV output leads, so may affect impedance balance to earth in some cases. Equipment shall therefore be tested while connected to a power supply of the type supplied to customers.
NOTES:
1. Resistors 'R' to be matched to within 0.1%
2. Capacitors 'C' to be matched to within 10%
3. Power earth & signal earth shall be connected together for duration of test
4. Balance Ratio to Earth = 20 log10 V1/V2 dB
5. Balance ratio to be determined with device in both 'on-hook' and 'off-hook' conditions.
FIG. 4-6 MEASUREMENT OF IMPEDANCE BALANCE RATIO TO EARTH
Return to Contents
4.9
Frequency response
4.9.1
Send frequency response of telephony devices
(1) The send frequency response of telephony devices connected to a zero length line, with the line current set at 20 mA, should preferably be within the limits specified below and shown in Fig. 4-7.
* The purpose is to ensure that the telephone is not excessively sensitive or excessively insensitive at particular frequencies. This is done by controlling the shape of the mask.
(2) The coordinates of the send frequency response mask, relative to the mid-point of the mask at 1 kHz are as follows:-
| Frequency (Hz) | Lower Limit (dB) | Upper Limit (dB) |
|---|---|---|
| (100) | - | (-12) |
| 200 | - | 0 |
| 300 | -16.5,-25 | - |
| 1000 | -3 | - |
| 2000 | -3 | +15 |
| 3000 | -8,-25 | +15 |
| 4000 | - | +15 |
| (5000) | - | (0) |
* The values shown are not absolute values but arbitrary relative levels with respect to the mid-point of the mask at 1 kHz. Compliance is checked by plotting frequency response on the same scales as the mask and adjusting the mask for "best fit".
* The figures in parenthesis are shown for guidance only and indicate the preferred performance at 100 Hz and 5000 Hz.
* Poor frequency response at the higher frequencies can result in poor articulation and the send mask is purposely sloped upwards to compensate for the frequency response of underground cables. Cables have increasing loss with increasing frequency.
FIG. 4-7 SEND FREQUENCY RESPONSE OF TELEPHONY DEVICES
(3) The send frequency response shall be measured. Return to Contents
4.9.2
Receive frequency response of telephony devices
(1) The receive frequency response of telephony devices connected to a zero length line, with the line current set at 20 mA, should be within the limits specified below and shown in Fig. 4-8.
(2) The coordinates of the receive frequency response mask, relative to the mid-point of the mask at 1 kHz, are as follows:-
| Frequency (Hz) | Lower Limit (dB) | Upper Limit (dB) |
|---|---|---|
| (100) | - | (-10) |
| 200 | - | +4 |
| 300 | -20, -25 | +4 |
| 500 | -4 | +4 |
| 1,000 | -4 | +4 |
| 2,000 | -4 | +4 |
| 3,000 | -12, -25 | +4 |
| 4,000 | - | +4 |
| (5,000) | - | (-2) |
* Compliance is checked by plotting frequency response on the same scales as the mask and adjusting the mask for "best fit". The limit values shown are not absolute values but arbitrary relative levels with respect to the mid-point of the mask at 1 kHz.
* The figures in parenthesis are shown for guidance only and indicate the preferred performance at 100 Hz and 5000 Hz.
* The receive response mask is deliberately 'flat' , the cable response being totally compensated for by the send response mask.
(3) At 8 kHz the receive sensitivity should be not greater than -15 dBPa/V when measured on a zero length line.
* This is a preferred requirement for any telephony device which is likely to be connected to a PABX or other digitally encoded customer premises equipment using 8 kHz sampling.
(4) The receive frequency response and 8 kHz sensitivity shall be measured.
FIG. 4-8 RECEIVE FREQUENCY RESPONSE OF TELEPHONY DEVICES
Return to Contents
4.9.3
Non-voice equipment
(1) The frequency range normally available over the Telecom PSTN for the transmission of data is 300 - 3400 Hz (ref Document TNA 102, clause 8.1).
(2) Subject to the power limits stated in clause 4.2, any equipment may transmit frequencies in the range 0 - 4000 Hz, but there is little likelihood that the network will pass signals over the whole of that frequency range.
* Signal frequencies above 4 kHz are treated as noise and shall satisfy the requirements detailed in clause 4.3.2.
(3) Any non-voice device which transmits operational signals outside the basic 300 - 3400 Hz band shall have the capability of automatically reducing the transmitted signal bandwidth until it operates successfully. A suitable warning notice shall be inserted in the User Instructions for such devices (see also clause 2.11.1(6) and (7)) .
* An example of this type of equipment is a 33.6 kbit/s data modem, capable of reducing its operational bit rate until successful transmission is obtained.
* A typical warning notice is given in Section 13 under clause 2.11.1. Return to Contents
4.9.4
Series connected devices
The frequency response requirements of series connected devices over the frequency range 300 - 3400 Hz are as follows:-
(a) 2-wire series connected devices in the non-terminating state shall have an insertion loss of no more than 1 dB at all frequencies, when the indirectly connected terminal device is off-hook and connected to line (see Fig. 4-4).
(b) 4-wire series connected devices in the non-terminating state shall have an insertion loss of no less than 2 dB and no more than 3 dB at all frequencies,
when the indirectly connected terminal device is off-hook (see Fig. 4-5).
* 2-wire/4-wire/2-wire devices normally have gain facilities but there is a need to guard against instability within the product when the 2-wire ports are connected to line or other equipment. To achieve this without unduly penalising the attenuation, an insertion loss of 2-3 dB is specified.
(c) The objective maximum insertion loss for series connected devices is 1 dB at all frequencies but, such is the variety of devices in this category that this is not a mandatory requirement. Each case will be treated on its individual merits.
(d) Series connected devices set up for through transmission, but which provide a line termination, shall have an insertion loss of no more than 3 dB at all frequencies in the off-hook condition.
* 1 dB insertion loss will be required wherever it is considered reasonable to do so.
* Many series connected devices are capable of performing a 'terminating' function and a 'non-terminating' function at different times during the same call (ref. clause 2.12). For this reason the terms are used to describe the state rather than the device, even though they can serve to describe some devices. Return to Contents
4.9.5
Bridging devices
The maximum insertion loss for bridging devices shall be 1 dB at all frequencies in the range 300 - 3400 Hz.
4.10
Send and Receive Loudness Ratings (SLR & RLR)
(1) The transmission performance of telephony devices shall be assessed in terms of ITU-T IRS Loudness Ratings in accordance with Recommendations P. 64:1993,
P. 65:1993 and P. 79:1993. The test arrangements are given in Appendix 2.
(2) Loudness ratings shall be assessed by taking measurements at 14 separate frequencies and weighting the results appropriately. For 14 frequency measurements the frequencies and their weightings are given in Table 1 of Appendix 2.
FIG. 4-9 SEND LOUDNESS RATING (SLR)
FIG. 4-10 RECEIVE LOUDNESS RATING (RLR)
(3) The aim is that, if a telephony device is set to the SLR and RLR limits stated below, when connected to the Telecom PSTN the combined performance of that device and the network will satisfy ITU-T Rec. G.111:1993 and Rec. G.121:1993.
* Reference also Specification TNA 151, Issue 2.
(4) (a) The send and receive loudness ratings at all line currents, with the telephone connected directly to the feed bridge, shall be within the following limits:-
SLR(dB) RLR(dB) Minimum (Loudest) +2 -10 (-14 dB if adjustable volume control is available) Maximum (Quietest) +11 -1
(b) There should be no more than 3 dB difference between the maximum and minimum send loudness rating results measured at line currents of 20 mA, 45 mA and 80 mA.
* The 3 dB recommended maximum difference reflects the preference for non-regulated telephones to be used.
(5) To assist suppliers to optimise performance, the following is the ideal design target area for SLR and RLR with the telephone connected directly to the feed bridge:-
| SLR(dB) | RLR(dB) | |
|---|---|---|
| 0 km Line | +5.0 ±1 | -6.5 ±1 |
* It should be noted that these design target area limits are not test limits for production samples. They are given here for guidance only.
(6) SLR and RLR shall also be tested using an artificial line (see Fig. 4-1) of lengths 3 km and 6 km. This is required primarily for comparison with overseas test results and for the assessment of more complex overseas products.
(7) Figures 4-9 and 4-10 show, for guidance only, the range of loudness ratings using an artificial line that can be expected for products fully complying with sub-clause (4)(a). The coordinates for these are as follows:-
| SLR(dB) | RLR(dB) | |
|---|---|---|
| 0 km Line (Min) | +2 | -14 |
| 0 km Line (Max) | +11 | -1 |
| 3 km Line (Min) | +6 | -10 |
| 3 km Line (Max) | +15 | +3 |
| 6 km Line (Min) | +10 | -6 |
| 6 km Line (Max) | +19 | +7 |
* These are not mandatory requirements, but a device fully compliant with sub-clause (4)(a) is likely to have a performance within these ranges.
* The above 6 km (Max) SLR limit is outside that given by ITU-T Recommendations G.111:1993 and G.121:1993. However, this is a test condition only, and is not representative of a practical situation used by Telecom. * Clause 4.9 is a change from Specification PTC 202:1988 in which the limits indicated a preference for 'regulated' telephones. The increasing use of low (around 20 mA) current line feeds means that 'regulated' telephones become deregulated. This results in excessively loud performance characteristics when used on shorter length lines. Return to Contents
4.11
Sidetone for telephony devices
4.11.1
Significance of sidetone
(1) Sidetone is an important factor in telephony and can have as much effect on the perceived quality of a telephone conversation as the actual loudness and clarity of the speech signals.
(2) Sidetone performance results from human head characteristics in combination with the mechanical and electro-acoustical properties of the telephone. Some sidetone is important to avoid "deadness" on a telephone, but excessive levels of sidetone have a marked effect not only on the perception of received signals, but also on the volume of the speaker's voice. Thus, excessive sidetone in one telephone can have a marked effect on the received speech volume of the telephone at the other end of a call.
(3) A high level of sidetone also has a significant effect on the use of telephones in high ambient noise conditions, making received speech extremely difficult to understand.
* Sidetone is related to SLR and RLR and any attempt to change them for telephones to achieve compliance with loudness rating requirements can also have a marked effect on sidetone performance.
* Reference ITU-T Recommendations G.121:1993 and P. 79:1993, and also CCITT Blue Book, Recommendation P. 76. Return to Contents
4.11.2
Optimum sidetone performance
(1) The internationally recognised method for assessing sidetone is by use of sidetone masking rating (STMR). The "preferred range" of STMR for a telephone on a complete call connection is 7 - 12 dB.
* Reference ITU-T Recommendation G.121:1993.
(2) For optimum sidetone performance in the Telecom network, with the objective of being within the "preferred range" stated in (1) above, the equivalent sidetone balance impedance of a telephone (Zs0), should be close to the line terminating impedance "BT3" (ref. Fig. 4.3).
* Accurate determination of STMR of a telephone for all types of complete connections is totally impractical due to the number of variables involved. However, assessment of STMR against the Telecom network terminating impedance "BT3" does provide some measure of sidetone performance.
* Zs0 is also sometimes known as the "minimum sidetone line impedance".
* Traditionally, the Zs0 of telephones has been considerably more reactive than BT3 in order to match more closely the combination of long cable pair connections in the local loop and the exchange impedance. This is no longer favoured due to the rapidly increasing number of short cable connections being used. Return to Contents
4.11.3
(2) The value shall be no less than 7 dB against BT3.
Sidetone Masking Rating (STMR)
(1) STMR shall be measured separately against the two impedances, 600 ohms and the Telecom network line terminating impedance BT3.
(3) It is preferred that STMR also be no less than 7 dB against 600 ohms.
* Measurement against two different impedances provides a better understanding of the likely sidetone performance when installed in practical conditions.
* This test requirement does not include use of an artificial line. Return to Contents
4.12
(2) Telephony devices shall be tested using the test set-up shown in Fig. 4-1, with the handset placed face down on a hard flat surface, it shall not be possible under any
condition to induce an audible oscillation in the telephone when terminated by the following impedance networks:-
(a) a 270 ohm resistor.
(b) a 1600 ohm resistor in series with a 510 nF capacitor.
(3) The requirement of sub-clause (2) above applies to all telephony devices, including those fitted with a receive volume control when adjusted to both minimum and maximum settings.
Instability
(1) All devices shall be stable under any normal range of conditions when connected to a PSTN line.
* Instability not only affects the user, but will also upset the other person taking part in the conversation. Return to Contents
4.13
(a) For handsets - max 118 dBA (approximately equivalent to +24 dBPa)
(b) For headsets - max 111 dBA
Acoustic shock protection
The sound pressure at the earpiece of telephony products shall not exceed the following limits when a 1000 Hz sinusoidal signal of up to +24 dBV (15.85 V) is applied via a 600 ohm resistor to the line terminals. The equipment shall comply at all line currents between 20 mA and 80 mA, and all signal generator voltages up to +24 dBV:-
* This requirement relates to the safety of the user and reduces the risk of line transients resulting in injury to the user's hearing.
4.14
Adjustable volume controls on telephony devices
(1) The requirements of clauses 4.2 to 4.13 shall be tested at both the "minimum" and "maximum" settings of all telephony products equipped with a receive volume control.
* These tests are carried out at "minimum" and "maximum" as they are normally the only readily discernable settings for a continuously variable volume control. The intention is to demonstrate that the two extremes do, in fact, fit within the specified performance range.
(2) When the volume control is set to maximum, the receive loudness rating shall be no louder than -14 dB at any line current (ref. clause 4.10(4)).
(3) It is strongly recommended that the volume control resets to a default setting when the device is restored to the on-hook state. This default setting should be such that the receive loudness rating resets to within the 'normal' range -1 dB to -10 dB (ref. clause 4.10(4)).
(4) It is preferable that the requirements ofclause 4.10(4)(b) be complied with at both minimum and maximum settings of the volume control.
(5) User-controlled volume controls shall not affect compliance with either the d.c. voltage or the send performance characteristics.
(6) The frequency response of a telephone in this class should preferably lie within the receive frequency requirements of clause 4.9.2 for both the minimum and the maximum setting of the volume control. Return to Contents
4.15
Telephony security in on-hook condition
(1) The security of telephony devices between the transmitter and line while the device is on-hook is adequately covered by clause 4.3.1(3).
(2) Additionally, no component or combination of components on a telephony device shall be capable of reproducing audible or intelligible speech while that device is on-hook, and there is conversational speech present from another device connected to the same line.
* An example of this is a ringing detector which responds to speech signals present on the line while the device is on-hook.
* This requirement relates to potential eavesdropping, whereby the telephone in its on-hook condition could provide a source of signals to an unlawful listener. Return to Contents
4.16
Recorded message quality
(1) The speech quality of any recorded message transmission shall be suitably intelligible to the party on the other end of the call.
* Reference clause 4.2 for power level of stored message transmissions.
* Reference clause 4.3 & clause 4.4 for noise and distortion requirements.
(2) Overall speech quality will be assessed subjectively by the testing authority and, in any cases of doubt, finally assessed by Access Standards at the time of application.
* A suitable quantitive means of assessment may be introduced at some future date.
* Quantitive or subjective 'mean opinion score' assessment of speech quality is extremely complex and would be costly to the applicant. For this reason, no formal qualitative requirement is stated in this Specification.
(3) Telecom reserves the right to re-assess the quality of the recorded speech transmission of any product after the grant of a Telepermit if there is evidence of substantial complaints from customers.
(4) If, during such a re-assessment, it is suitably established that the transmitted speech quality is unintelligible to another party when connected via the Telecom PSTN, then either:-
(a) the speech quality shall be improved to the satisfaction of Telecom, or
(b) the Telepermit will be cancelled.
(5) The cost of such re-assessment shall be born by the Telepermit holder. Also, any cost incurred by Telecom for investigation of complaints by customers relating to transmitted speech quality may be passed on to the customer using the transmitting device.
